The invention relates to a method for breaking a silicon body and to a polycrystalline silicon portion.
In particular, the invention relates to a method for breaking polysilicon rods.
Polycrystalline silicon (polysilicon) is conventionally produced by vapor deposition in a Siemens reactor. Highly pure silane or chlorosilane is in this case deposited on a heated substrate (preferably consisting of silicon) so that solid rods, blocks or slabs are obtained.
Before this polysilicon can be used in crystallization methods, it must be reduced in size. During the ultrapure silicon production process, in various process steps it is necessary to break silicon rods of different cross sections and lengths, in order to introduce them as rod portions or fragments (chips, chunks) as a starting material into other production steps, either directly or after subsequent grinding processes.
In the prior art, in a first step a silicon rod in the form obtained from a Siemens deposition reactor is prebroken using a hand hammer and subsequently manually reduced further to the required fraction size using a rivet hammer. This manually produced coarse fraction as feed material may be further reduced by machine, for example using a jaw crusher or a roller crusher.
The size reduction of relatively long rods, which may be from 2 to 3 m long, should be carried out in such a way that the basic material, i.e. ultrapure silicon, is contaminated as little as possible by the action of mechanical forces and the least possible material losses (fine fraction) are incurred.
When breaking relatively long rods, the breaking tools used should not transfer any unacceptable abrasion particles into the contact surfaces between the tool and the workpiece, and they should act with the lowest possible forces.
Conventional machine breaking methods of construction or mining require high impact forces and hard cutting tool materials.
These always contain material components which are undesirable even in traces for ultrapure silicon applications, because they cause quality problems in further processing steps or directly influence physical semiconductor properties (for example by dopants).
U.S. Pat. No. 7,950,308 B2 discloses a hammer for the manual breaking of polysilicon. The hammer head consists of hard metal. This type of preliminary size reduction leads to undesired contamination with metals, particularly on the surfaces around the region of the impact of the hammer, including the fracture surface, i.e. a new surface which is exposed on a fragment of the previous whole. Furthermore, material losses in the form of fine fragments are incurred.
DE 10 2008 024 411 A1 describes the size reduction of a starting material comprising silicon using pulsed shockwaves: silicon particles having a diameter of from 0.1 μm-1 cm are obtained. For larger fragments or the production of rod fragments, the method is unsuitable.
DE 42 18 283 A1 discloses a method for the contamination-free size reduction of semiconductor material, in particular silicon, the semiconductor material being exposed to shockwaves. Shockwaves are transmitted onto the semiconductor material through water as a transfer medium. The generation of the shockwaves may be carried out at the focal point of a semi-ellipsoid reflector by electrical discharge between two electrodes. Fragments of different size are obtained.
Expediently, the semiconductor material is exposed to shockwaves until a desired limit size of the fragment is reached or fallen below.
For preliminary size reduction or the generation of rod portions of defined size, the method is not suitable.
DE 197 49 127 A1 describes a method of preparing for the size reduction of a crystal in rod form, an impulse which has an impact effect being transmitted onto the crystal. Mechanical striking tools or shockwaves are used in this case. The impulse may be transmitted through a liquid jet. Microcracks are then formed, which are intended to facilitate subsequent breaking.
The size reduction of a silicon rod by thermal shock is likewise known. To this end, JP3285054 A uses laser heating. A disadvantage with this method is that diffusion processes are initiated at high temperatures. In this case, at least some of the surface contaminations, in particular extraneous metals, enter the bulk of the rod or the fragments and are removed from subsequent surface cleaning/etching.
All preliminary size reduction steps known from the prior art are thus associated with high energy consumption, sometimes high equipment outlay, but above all loss in the form of fine fragments as well as undesired contamination of the workpiece to be broken.
The object of the invention is based on this problem.